1. Field of the Invention
This invention relates, generally, to a catalyst system. Specifically, this invention relates to hemiisospecific catalysts.
2. Description of Related Art
Olefins, especially propylene, may be polymerized to form polyolefins in various forms: isotactic, syndiotactic and atactic. Isotactic polypropylene contains principally repeating units with identical configurations and only a few erratic, brief inversions in the chain. Isotactic polypropylene may be structurally represented in a Fischer projection as 
In Bovey""s NMR nomenclature the isotactic structure is designated . . . mmmm . . . since the five successive methyl groups are meso to each other, i.e., on the same side of the plane in a Fischer projection.
Isotactic polypropylene is capable of being a highly crystalline polymer with a high melting point and other desirable physical properties that are considerably different from the polymer in an amorphous (noncrystalline) state.
A syndiotactic polymer contains principally units of exactly alternating stereoisomers and is represented in a Fischer projection by the structure: 
In Bovey""s NMR nomenclature the syndiotactic structure is designated . . . rrrr . . . since the five successive methyl groups are racemic to each other, i.e., on alternate sides of the plane in a Fischer projection.
A polymer chain showing no regular order of repeating unit configurations is an atactic polymer. In commercial applications, a certain percentage of atactic polymer is typically produced with the isotactic form.
There are other variations in the form of polymer structure. Hemiisotactic or hemitactic polypropylene was disclosed in xe2x80x9cHemitactic Polypropylene: An Example of a Novel Kind of Polymer Tacticityxe2x80x9d by M. Farina, G. Di Silvestro and P. Sozzani (Macromolecules, Vol. 15, 1451-1452, 1982). The structure of hemiisotactic polymers is represented in a Fischer projection as follows: 
The monomeric unit of the polymer is of the following structure: 
where RS is a hydrocarbyl group or nonhydrocarbyl group. The second carbon atom in formula (6) is the asymmetric carbon atom, i.e., the one which does not have identical groups attached, hence xe2x80x9casymmetricxe2x80x9d.
The structure of the polymer is characterized by RS groups attached to every other asymmetric carbon atom being on the same side of the principal polymer chain as represented in a Fischer projection and RS groups attached to the remaining asymmetric carbon atoms being either on the same side or the opposite side of the RS groups attached to every other asymmetric carbon atom. When RS groups are on the same side of the principal polymer chain, the structure is isotactic. Since only every other one conforms to the isotactic structure, it is xe2x80x9chemixe2x80x9d. The material is a noncrystalline polymer.
Polymerization of olefins is primarily with Zeigler-Natta catalysts one family of Zeigler-Natta catalysts is Group IV metallocene compounds with methylaluminoxane as a cocatalyst. German patent application No. 2,608,863 discloses a catalyst system for the polymerization of ethylene consisting of bis(cyclopentadienyl)titanium dialkyl, an aluminum trialkyl and water. German patent application No. 2,608,933 discloses an ethylene polymerization catalyst system consisting of zirconium metallocenes of the general formula (cyclopentadienyl)nZrY4xe2x88x92n, wherein Y represents R1CH2AlR2, CH2CH2AlR2 and CH2CH(AlR2)2 where R stands for an alkyl or metallo alkyl, and n is a number within the range 1-4; and the metallocene catalyst is used in combination with an aluminum trialkyl cocatalyst and water.
The use of metallocenes as catalysts in the copolymerization of ethylene and other alpha-olefins is also known in the art. U.S. Pat. No. 4,542,199 to Kaminsky, et al. discloses a process for the polymerization of olefins and particularly for the preparation of polyethylene and copolymers of polyethylene and other alpha-olefins. The disclosed catalyst system includes a catalyst of the formula (cyclopentadienyl)2MeRHal in which R is a halogen, a cyclopentadienyl or a C1-C6 alkyl radical, Me is a transition metal, in particular zirconium, and Hal is a halogen, in particular chlorine. The catalyst system also includes an alumoxane having the general formula Al2OR4(Al(R)xe2x80x94O)n for a linear molecule and/or (Al(R)xe2x80x94O)n+2 for a cyclic molecule in which n is a number from 4-20 and R is a methyl or ethyl radical. A similar catalyst system is disclosed in U.S. Pat. No. 4,404,344.
U.S. Pat. No. 4,530,914 discloses a catalyst system for the polymerization of ethylene to polyethylene having a broad molecular weight distribution and especially a bimodal or multimodal molecular weight distribution. The catalyst system is comprised of at least two different metallocenes and an alumoxane. The patent discloses metallocenes that may have a bridge between two cyclopentadienyl rings with the bridge serving to make those rings stereorigid.
European Patent Publication No. 0185918 discloses a stereorigid, chiral zirconium metallocene catalyst for the polymerization of olefins. The application does not indicate that hafnium could be substituted for the zirconium and used to produce a useful polymer product. The bridge between the cyclopentadienyl groups is disclosed as being a linear hydrocarbon with 1-4 carbon atoms or a cyclical hydrocarbon with 3-6 carbon atoms.
European Patent Application 0-277-003 relates to work by Turner on a catalyst prepared by a protonation method. A bis(cyclopentadienyl) metal compound is combined with a compound having a cation capable of donating a proton and an anion having a plurality of boron atoms. For example, the following reaction illustrates the invention:
bis(cyclopentadienyl)hafnium dimethyl+N,N-dimethylanilinium bis(7,8-dicarbaundecaborato) cobaltate(III) xe2x86x92[Cp2HfMe][B]+CH4+N,N-dimethylaniline
where [B] is 7,8-dicarbaundecaborane.
European Patent Application 0-277-004 also relates to work by Turner on a catalyst prepared by a protonation method. A bis(cyclopentadienyl) metal compound is combined with an ionic compound having a cation which will irreversibly react with a ligand on the metal compound and an anion having a plurality of lipophilic radicals around a metal or metalloid ion. For example, the following reaction illustrates the invention:
xe2x80x83tri(n-butyl)ammonium tetra(pentafluorophenyl)boron+bis(cyclopentadienyl)zirconium dimethylxe2x86x92[Cp2ZrMe][BPh4]+CH4+(n-Bu)3N
A system for the production of isotactic polypropylene using a titanium or zirconium metallocene catalyst and an alumoxane cocatalyst is described in xe2x80x9cMechanisms of Stereochemical Control in Propylene Polymerization with Soluble Group 4B Metallocene/Methylalumoxane Catalysts,xe2x80x9d J. Am. Chem. Soc., Vol. 106, pp. 6355-64, 1984. The article shows that chiral catalysts derived from the racemic enantiomers of ethylene-bridged indenyl derivatives form isotactic polypropylene by the conventional structure predicted by an enantiomorphic-site stereochemical control model. The meso achiral form of the ethylene-bridged titanium indenyl diastereomers and achiral zirconocene derivatives, however, produce polypropylene with a purely atactic structure.
In accordance with the present invention, there is provided a metallocene compound having a general formula of 
where each R and Rxe2x80x2 is a hydrocarbyl radical having from 1-20 carbon atoms, is the same or different and is selected such that CpRxe2x80x2m is a sterically different ring from CpRn resulting in a lack of bi-lateral symmetry for the compound, Rxe2x80x3 is a structural bridge imparting stereorigidity to the compound, M is a Group 4 metal, n is from 0 to 4, m is from 0 to 4 and Hal is a halogen. One example of such a compound is isopropylidene (3-methylcyclopentadienyl-1-fluorenyl)zirconium dichloride. This compound is a bridged, metallocene compound having dissimilar cyclopentadienyl groups and no bi-lateral symmetry.
One use for these compounds is in a metallocene catalyst system. The metallocene compounds defined above can be activated as catalysts by any known method of metallocene catalyst preparation
The polymer produced with the catalyst of this invention has the structure termed xe2x80x9chemiisotacticxe2x80x9d. Hemiisotactic polypropylene is characterized by every other methyl group being on the same side of the principal polymer chain as represented by a Fischer projection. The remaining methyl groups can be either on the same side or the opposite side of the principal polymer chain.
Propagation of the polymer chain results from head-to-tail linkage of the propylene monomer units in such a way the following structure is formed: 
In this Fischer projection representation the odd numbered methine units are meso with respect to each other and the even numbered methine carbons have random steric configurations. Hemiisotactic polypropylene is noncrystalline due to the disorder and irregularity of these random groups.
The invention is for a new metallocene compound which is a catalyst precursor for a catalyst used to produce polymers termed hemiisotactic. The metallocene compound is changed to a metallocene catalyst with an ionizing agent which converts the neutral metallocene compound to a metallocene cation which operates as a catalyst. The ionizing agent can be a cocatalyst compound such as methylaluminoxane (MAO).
A preferred application of the invention is in the hemiisotactic polymerization of monomers which may be characterized in terms of the following formula:
xe2x80x83CH2xe2x95x90CHxe2x80x94RSxe2x80x83xe2x80x83(7)
wherein RS is a hydrocarbyl group or nonhydrocarbyl substituent. Monomers to which the present invention is applicable are C3+ alpha olefins, 1-butene, 1-dienes, such as 1,3-butadiene, substituted vinyl compounds, such as vinyl chloride, and styrene. The preferred application is to ethenically unsaturated monomers. By the term xe2x80x9cethenically unsaturated monomerxe2x80x9d as used herein is meant a hydrocarbon or substituted hydrocarbon compound characterized by a terminal vinyl group (CH2xe2x95x90CHxe2x80x94). The most preferred ethenically unsaturated compounds employed in the present invention have at least three carbon atoms. A specific example is propylene.
The catalyst used to produce hemiisotactic olefins is from a metallocene compound having a general formula of
Rxe2x80x3 (CpRn) (CpRxe2x80x2m)MHal2xe2x80x83xe2x80x83(8)
where Cp is cyclopentadienyl or substituted cyclopentadienyl, each R and Rxe2x80x2 is a hydrocarbyl radical having from 1-20 carbon atoms and is the same or different and is selected such that CpRxe2x80x2m is a sterically different ring from CpRn resulting in a lack of bi-lateral symmetry for the compound, Rxe2x80x3 is a structural bridge imparting stereorigidity to the compound, M is a Group 4 metal, preferably titianium, zirconium or hafnium, n is from 0 to 4, m is from 0 to 4 and Hal is a halogen, preferably chlorine.
The lack of bi-lateral symmetry for the compound is defined as the condition in which a metallocene compound having one non-cyclopentadienyl coordination site has no substituents or one or more substituents on one side of the cyclopentadienyl rings both above and below the coordination site and one or more substituents on the other side of the cyclopentadienyl rings either above or below the coordination site. One example of such a compound is isopropylidene(3-methylcyclopentadienyl -1-fluorenyl)zirconium dichloride, abbreviated iPr(3MeCp-1-Flu)ZrCl2. An illustration of the ligand of this compound is shown below: 
The lack of bi-lateral symmetry is illustrated by the right side of the drawing being different from the left because one methyl group is on the right side of one cyclopentadienyl ring and no substituents are on the left side of the same cyclopentadienyl ring.
The iPr(3MeCp-1-Flu)ZrCl2 compound was prepared by cracking the methylcyclopentadiene diner, preparing 3,6,6-trimethylfulvene, bridging the two cyclopentadiene compounds with an isopropylidene bridge and forming a coordination compound with zirconium and chlorine. Final reactions were carried out in tetrahydrofuran (THF) and in methylenedichloride (MeCl2), also known as dichloromethane. Use of MeCl2 allows the iPr(3MeCp-1-Flu)ZrCl2 to be isolated in pure form.
Polymerization of the olefin is accomplished by any of the known means for polymerization of olefins with metallocene catalysts, for example polymerization in bulk, slurry or gas phase. For polypropylene, polymerization temperatures range from xe2x88x9280xc2x0 C. to 150xc2x0 C., preferably 25xc2x0 C. to 90xc2x0 C. and most preferably from 50xc2x0 C. to 80xc2x0 C.
The noncrystalline hemiisotactic polypropylene has use as a plasticizer for syndiotactic or isotactic polypropylene. A plasticizer is a material incorporated in a plastic to increase its workability and its flexibility or distensibility. The addition of a plasticizer may lower the melt viscosity, the temperature of the second-order transition, or the elastic modulus of the plastic. The plastic and plasticizer are intimately mixed which is most commonly done by heating until the plastic has dissolved into the plasticizer of vice versa. Alternatively, the plastic and plasticizer are mixed by dissolution in a common solvent without heat followed by removal of the solvent by evaporation.
Hemiisotactic polymer is noncrystalline and with its partial stereoregular structure would have properties of a plasticizer. A specific example of a -hemiisotactic polymer as a plasticizer is a reactor blend of hemiisotactic polypropylene and syndiotactic polypropylene made by polymerizing propylene simultaneously with both iPr(3MeCp-1-Flu)ZrCl2 and isopropylidene (cyclopentadienyl -1-fluorenyl)zirconium dichloride, abbreviated iPr(Cp-1-Flu)ZrCl2, or any other syndiospecific catalyst precursor. A reactor blend of hemiisotactic and isotactic polypropylene is possible by polymerizing propylene simultaneously. with both iPr(3MeCp-1-Flu)ZrCl2 and ethylenebis(tetrahydroindenyl)zirconium dichloride, abbreviated Et(IndH4)2ZrCl2, or any other isospecific catalyst precursor. The amount of hemiisotactic polypropylene in mixture with isotactic or syndiotactic polypropylene can range from 1-90% by weight, depending on desired physical properties of the plasticized plastic. Preferably, the amount of hemiisotactic polypropylene in mixture with isotactic or syndiotactic polypropylene ranges from 5-50% by weight. Most preferably, the amount of hemiisotactic polypropylene in mixture with isotactic or syndiotactic polypropylene is approximately 10% by weight.